My sliding hatch was also very stiff, especially when wet if I recall correctly. It had also been damaged on the aft edge prior to my ownership, and had been glued back together. This winter the repair broke, and I decided to get a replacement from Houdini [who made the hatches for Parkers]. The new hatch slides very easily - Charlie Stewart from Houdini said they had to carefully polish the sliding surfaces when they make these hatches, so it may be that as the hatch accumulates scratches it gets more difficult to slide.
Might be worth asking Houdini if polishing the hatch would help.
I'm sure Houdini will also be able to identify, if not supply, the seal material for the sliding hatch.
Houdini's contact information is:-
Houdini Marine Windows Limited.
Hallmark Industrial Estate, Southminster, Essex, CM0 7EH
Tel: +44 (0) 1621 773590
Fax: +44 (0) 1621 773852
Email: enquiries@houdini-marine.co.uk
Dick
Dark Star P275 No 36